I think if you heat the piece lightly with torch, around 200 degrees
F or let it sit in a window in direct sunlight for an hour or so,
you'll have much better results. You could also increase the strength
of your solution. The piece should be warm enough that you can see
the soln. slowly steaming off. If you see it instantly boiling off
when you apply the soln., it is too hot and may not fully react.
Give it a shot and see if it helps.
Black Horse Design